Munin is commonly used to graph lots of systems stuff, however it lacks a common piece of functionality: 95th percentile.

The Munin bug tracker has ticket #443 sitting open for 7 years now, asking for this, and proving a not-great patch for it.

I really wanted to add 95th percentile to one of my complicated graphs (4 base variables, and 3 derived variables deep), but I didn't like the above patch either. Reading the Munin source to consider implementing VDEF properly, I noticed an undocumented setting: graph_args_after. It was introduced by ticket #1032, as a way of passing things directly to rrdtool-graph.

Clever use of this variable can pass in ANYTHING else to rrdtool-graph, including VDEF! So without further ado, here's how to put 95th percentile into individual Munin graphs, relatively easily.

# GRAPHNAME is the name of the graph you want to render on.
# VARNAME is the name of the new variable to call the Percentile line.
# DEF_VAR is the name of the CDEF or DEF variable from earlier in your graph definition.
# LEGEND is whatever legend you want to display on the graph for the line.
# FYI Normal rrdtool escaping rules apply for legend (spaces, pound, slash).
${GRAPHNAME}.graph_args_after \
VDEF:${VARNAME}=gcdef${DEF_VAR},95,PERCENT \
LINE1:${VARNAME}\#999999:${LEGEND}:dashes \
GPRINT:${VARNAME}:\%6.2lf\%s\\j